The present invention concerns a method for removal of H2S in drilling mud, and use of a relatively sparingly soluble divalent iron salt in drilling mud.
During drilling for oil and gas hydrogen sulphide (H2S) can come into drilling mud from formations or be formed from the drilling mud. H2S is both poisonous and corrosive, and it is therefore necessary to remove it from the drilling mud/fluid. Today zinc carbonate and iron oxide are used. These two mentioned metal compounds have, however, limitations. Zinc carbonate, which can be used in all types of drilling mud, is not friendly to the environment, and effluents of this are therefore prohibited many places in the world. Iron oxide can only control H2S at a lower pH than the pH which is usual in drilling mud.
Hydrogen sulphide (H2S) is produced by bacteria in sea water or in formation water (connate water) where the mentioned bacteria are growing, with organic material (for instance crude oil) as a substrate, under anaerobic conditions. The reaction can be described by the following reaction equation:
CH4+2H*+SO42xe2x88x92=H2S+CO2+2H2Oxe2x80x83xe2x80x83(1)
H2S is a colourless gas which is easily soluble in solvents like water, alcohols and hydrocarbons. Low concentrations of H2S in air smell like rotten eggs, but it is very important to be aware of that the smelling ability of humans is lowered at relatively low concentrations of the gas (100 ppm). When a water lock which has been closed for a longer period of time suddenly is opened, the characteristic smell of hydrogen sulphide can be recognised. Hydrogen sulphide has a density of 1.18 compared to air, which means that the gas is sinking against the lowest possible level, as for instance close to sludge tanks. The gas is forming an explosive mixture with air in concentrations of 4.3 to 45% H2S, which is especially dangerous compared to methane which is combustible at concentrations ranging from 5 to 16%. H2S has an ignition temperature of 260xc2x0 C. and methane of 538xc2x0 C. Hydrogen sulphide forms sulphur dioxide by combustion, and this is also a poisonous gas.
H2S will be present in drilling mud/water either as H2S, HSxe2x88x92, or S2xe2x88x92, depending on the pH-value. In drilling mud, which has a pH value from 7 to 13, bisulphide is dominating, which is water soluble arid not very dangerous for the environment. A common method of handling sulphide in drilling mud is therefore to increase the pH value with sodium hydroxide and/or lime. The hazard is, however, that by accumulation of soluble sulphide components in drilling mud, a drop in pH can liberate big amounts of H2S gas from the sludge. It is therefore practical that sulphide containing components are reacting to more inert forms as i.a. precipitated and insoluble metal sulphides. The mentioned reactions can take place either by oxidation reactions wherein the sulphide is converted to harmless sulphur compounds, or by reactions leading to precipitation of sulphide containing compounds. In the last mentioned case the oxidation state of the sulphide ions is not altered. Reaction precipitation can be described as follows:
MX⇄M+++X++xe2x80x83xe2x80x83(2)
M+++H2Sxe2x86x92MS↓+2H+xe2x80x83xe2x80x83(3)
The expression sulphide scavengers (sulphide absorption agents) comprises any chemical substance which is added and which can react with one or more sulphide components to form more inert compounds. It is appropriate to add sulphide absorption agents to the drilling fluid in excess in relation to the amount of H2S present or expected amount of H2S supplied, to secure a fast removal of sulphide containing components. Known additives based on zinc, such as zinc carbonate, zinc hydroxide and organic zinc compounds provide effective sulphide absorption means and give irreversible reactions with the sulphides under formation of solid zinc sulphide. It has been preferred to use zinc as a metal ion because of its properties with regard to i.a. solubility and precipitation in drilling mud.
U.S. Pat. No. 3,928,211 describes a method for removal of H2S in water containing drilling fluids and a number of known methods for the mentioned purpose. Zinc carbonate and zinc hydroxide are described as effective sulphide pickup substances. U.S. Pat. No. 4,252,655 comprises a method for removal of sulphide ions in both water and oil based drilling fluids with organic zinc chelates. U.S. Pat. No 3,928,211 and U.S. Pat. No. 4,252,655 are incorporated herein as references. It is as mentioned known to remove hydrogen sulphide during drilling for oil by adding zinc carbonate to the used drilling mud, but this is an environmentally unaccepted compound.
Additives such as flocculation and viscosifying agents are usually added to drilling fluids which can function as cleaning agents and thereby reduce the sulphide concentration with a few ppm. At normal shearing speeds and temperatures the sulphides can be oxidized to elementary sulphur by contact with air.
U.S. Pat. No. 4,756,836 comprises a method to reduce the amount of H2S in drilling mud by oil drilling, whereby iron chelate is added, preferably In the form of EDTA, HEDTA, NTA or DTPA. Drilling mud wherein iron chelate has been added is passed down into a well and brought in contact with hydrogen sulphide under formation of elementary sulphur.
Singh, A. K. et al describe the use of Iron oxides to remove H2S in drilling mud in the article  less than  less than Handling Hydrogen Sulphide in Drilling Fluids greater than  greater than , part 2. The iron oxides are, however, nearly insoluble both in water and drilling fluids.
U.S. Pat. No. 4,805,708 comprises a method for determination of the amount of unused zinc based cleaning agent during oil drilling. The invention comprises selective dissolution and extracting of unused cleaning agent in a solvent such as e.g. glacial acetic acid, separation of the water containing solution and thereafter determination of the concentration of dissolved zinc in the mentioned solution. U.S. Pat. No. 4,805,708 describes zinc based cleaning agent in the form of zinc carbonate, which, however, is defined as injurious to health.
The present invention concerns a method for removal of H2S by use of a relatively sparingly soluble divalent environmentally acceptable iron salt in both oil and water based drilling mud. It is of essential importance that the divalent iron salt which is used does not affect the properties of different types of drilling mud and that it does not have any negative environmental consequences. It is an important feature of the present invention that the divalent iron salt is sparingly soluble, but has a sufficient solubility in drilling mud to cause the present sulphide ions to precipitate as iron sulphide. Preferably the divalent iron salt is used in excess in relation to the amount of H2S in the drilling mud.
According to the present invention it is of essential importance that compounds used in drilling mud remove sulphide containing components, and that the substances are not harmful to the environment. Known art does not comprise methods for removal of hydrogen sulphide by adding a environmentally friendly and relatively difficulty soluble divalent iron salt as well as use of the mentioned metal compound. The present invention is therefore on the background of known art considered to involve both technical and environmental advantages.
According to the present invention it is preferred that the divalent iron salt which is used for removal of H2S in drilling mud is iron oxalate. Iron oxalate is friendly to the environment, has a low solubility, but is high enough in alkaline environments for removal of H2S in drilling mud. It is maintaining the absorption ability for H2S over time in drilling mud without altering the physical properties of the mud. The iron salts which are used to remove hydrogen sulphide must be able to be used at alkaline conditions, where H2S is present as sulphide ions in solution and not as a gas. Divalent iron oxalate has as known lower solubility than trivalent iron oxalate, but it was surprisingly found that a divalent iron oxalate has sufficient solubility, so that sulphide containing compounds react with the divalent iron ions, and thereafter precipitating as iron sulphides.
The present invention comprises a method for removal of H2S in drilling mud where a relatively sparingly soluble divalent iron salt is added to the drilling mud which is circulated in a borehole; whereby hydrogen sulphide which is present in the drilling mud reacts with a relatively sparingly soluble iron salt to iron sulphide.
The invention is characterized by the fact that the divalent iron salt is added in excess in relation to the amount of H2S which is present or expected to be taken up into the drilling mud. It is preferred that the iron salt is added in an excess from 10% and up to 10 times, especially that the iron salt is added in an excess of 50% in relation to the amount of H2S which is present or expected to be taken up into the drilling mud. One of the objects of the present invention is to provide a depot of a difficulty soluble divalent iron salt, which is gradually dissolved in the drilling mud when it is reacting with H2S. One of the advantages of the present invention is that the iron sulphide which has been formed can remain in the drilling mud without having influence on the mud. The divalent iron salt normally has a solubility of from 0.1 to 1000 ppm at room temperature in drilling mud, more preferred from 0.1 to 200 ppm and most preferred from 0.1 til 50 ppm. An especially preferred divalent iron salt is iron(II) oxalate. The drilling mud normally has a pH value from 8 to 12.5. Further the invention comprises the use of a relatively sparingly soluble divalent iron salt for removal of H2S in drilling mud. This iron salt is preferably iron(II) oxalate.